Testing the Supernova , Cepheid , and Early - type Galaxy Distance Scales

I summarize recent work comparing relative distances measured to individual galaxies with independent methods. The comparisons include: ground-based surface brightness fluctuation (SBF) and fundamental plane distances to 170 galaxies, distances predicted from galaxy velocities and the inferred gravity field, HST SBF measurements to seven early-type hosts of Type Ia supernovae, and ties of the Cepheid distance scale to early-type galaxies. Independent calibrations for some methods provide interesting constraints on the Cepheid zero point. The two most frequently applied early-type galaxy distance indicators are the fundamental plane (FP, and the related D n-σ) and surface brightness fluctuations (SBF) methods. In a recent study (Blakeslee et al. 2001, 2002), we used V-and I-band data from the ground-based SBF Survey (Tonry et al. 2001) to calculate FP photometric parameters for 170 galaxies with velocity dispersions available in the homogenized SMAC catalogue (Hudson et al. 2001). To our knowledge, this is the largest galaxy-by-galaxy comparison of different standard candle/rod distance methods to date. Fig. 1a shows the comparison. Overall the distance agreement was good, but several low-luminosity, S0 galaxies had systematically low FP distances, probably due in part to younger ages and lower mass-to-light ratios, although aperture effects may also contribute. The SBF distances are tied to the Cepheids via measurements in spiral bulges, while the FP distances are tied to the Hubble flow via distant clusters; the Hubble constant that results from this comparison is H 0 = 68 km s −1 Mpc −1. However, we also derived independent distances for these galaxies based on their velocities and the gravity field inferred from the redshift-space galaxy density; the resulting comparison with SBF yields H 0 = 74 (Fig. 1b), formally discordant at the 2 σ level with the FP-SBF result, but within the range of the systematic uncertainties in the various ties. Another interesting facet of this work relates to the " fluctuation number " N ≡ m − m tot , which measures the galaxy luminosity in units of the weighted mean stellar luminosity. N correlates tightly with stellar velocity dispersion; it also correlates with galaxy color and is independent of Galactic extinction. Interestingly, SBF distances calibrated using the properties of N , such as those shown in Fig. 1b, can be viewed as a hybrid of SBF and FP distances, and may be more accurate than those calibrated from galaxy color alone. We plan to investigate these issues in more detail.